*2.2. Sputtering Methods*

#### 2.2.1. DC and RF Sputtering

Several sputtering methods as well as combinations of different sputtering methods have been used to deposit both CrAl-based and AlCr-based coatings from elemental or composite cathodes. Even special processes based on the combination of composite targets combined with elemental targets have been realised [41]. The first CrAl-based coatings were deposited by reactive DC magnetron sputtering of a composite target (Cr75Al75) [14,15]. Additionally, newer research on a laboratory scale as well as industrial-scale PVD systems are using DC magnetron sputtering [41–46]. It should be noted that special magnetic field configurations can be achieved within the PVD systems by means of the magnetron set-ups, e.g., the CFUBM (Closed Field Unbalanced Magnetron Sputtering) [47,48]. RF (Radio Frequency) sputtering has been used for basic investigation [49,50]. Even the combination of RF sputtering plus DC sputtering has been applied [51,52], though RF sputtering is typically not used for industrial applications.

#### 2.2.2. Standard Pulsed Sputtering and HiPIMS

Besides standard pulsed magnetron sputtering [41], the mode of dual pulsed magnetrons has also been used [48]. The most advanced pulsed sputtering method is highpower impulse magnetron sputtering. Pulses with peak power densities in the range of several hundred W/cm<sup>2</sup> up to several thousand W/cm2 are achieved in classic high-power impulse magnetron sputtering (HiPIMS), also referred to as HPPMS, MPP or HIPAC. The pulse duration is usually in the range of 25 to 250 μs [41,53–56]. The longest pulse lengths of around 1000 μs are used in the case of MPP [57].

A dedicated HiPIMS process, known as the S3p process, is a process that runs with a constant pulse current and can operate using longer pulses than the classic HiPIMS process. The pulse duration often lies in the range of several milliseconds [30].
